A communication scheme succeeding to W-CDMA and HSDPA, that is, LTE (Long Term Evolution), is discussed in a standardization group 3GPP for the W-CDMA. As radio access schemes for the LTE, OFDM (Orthogonal Frequency Division Multiplexing) and SC-FDMA (Single-Carrier Frequency Division Multiple Access) are discussed for downlink and uplink, respectively. For example, see 3GPP TR 25.814 (V7.0.0), “Physical Layer Aspects for Evolved UTRA”, June 2006.
The OFDM is a scheme where a frequency band is divided into smaller frequency bands (subcarriers) and data is transmitted in these frequency bands. According to the OFDM, it is possible to achieve faster transmission and improve frequency utilization efficiency by arranging the subcarriers in the frequency band densely without mutual interference while allowing overlapping of the subcarriers.
The SC-FDMA is a scheme where a frequency band is divided and the resulting frequency bands are used for different terminals, which can reduce interference among the terminals. According to the SC-FDMA, variations of transmit power can be reduced, resulting in less consumption of power in the terminals and wider coverage.
In the LTE system, a base station (eNB: eNodeB) controls receive timings of signals received in uplink channels from simultaneously accessing mobile stations such that the receive timings can be within a CP length. In addition, the base station controls transmit timings of the mobile stations through downlink signaling in order to maintain orthogonality among the mobile stations.
In the case where a mobile station is currently communicating to the base station, the base station may control the transmit timing based on the receive timing of a pilot signal transmitted from the mobile station. In the case where a mobile station attempts to start communications to the base station, on the other hand, the base station may control the transmit timing based on the receive timing of a RACH (Random Access Channel) transmitted from the mobile station.
For the above downlink signaling, an amount of the transmit timing adjusted in the mobile station may be defined as follows,the adjustment amount=(minimum unit of the adjustment amount)×k, where k is −K1, −K1+1, . . . , −1, 0, 1, . . . , K2−1, K2.
In the formula, −K1 through K2 corresponds to a range of the adjustment amount of the transmit timing, and as that range is wider, a greater number of bits is required for the downlink signaling.